In enzymology, an icosanoyl-CoA synthase (EC 2.3.1.119) is an enzyme that catalyzes the chemical reaction

stearoyl-CoA + malonyl-CoA + 2 NAD(P)H + 2 H⁺ ${\displaystyle \rightleftharpoons }$ icosanoyl-CoA + CO₂ + CoA + 2 NAD(P)+ + H₂O

The 5 substrates of this enzyme are stearoyl-CoA, malonyl-CoA, NADH, NADPH, and H⁺, whereas its 6 products are icosanoyl-CoA, CO₂, CoA, NAD⁺, NADP⁺, and H₂O.

This enzyme belongs to the family of transferases, specifically those acyltransferases transferring groups other than aminoacyl groups. The systematic name of this enzyme class is stearoyl-CoA:malonyl-CoA C-acyltransferase (decarboxylating, oxoacyl- and enoyl-reducing). Other names in common use include acyl-CoA elongase, C18-CoA elongase, and stearoyl-CoA elongase.

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• Bessoule JJ, Lessire R, Rigoulet M, Guerin B, Cassagne C (1987). "Fatty acid synthesis in mitochondria from Saccharomyces cerevisiae". FEBS Lett. 214 (1): 158–62. doi:10.1016/0014-5793(87)80033-9. PMID 3552725.
• Lessire R, Bessoule JJ, Cassagne C (1985). "Solubilization of C18 CoA and C20 CoA elongases from Allium porrum L. epidermal-cell microsomes". FEBS Lett. 187 (2): 314–320. doi:10.1016/0014-5793(85)81267-9.